Replace zerowidth invisible chars with a zero-advance space glyph

Like Uniscribe does.

src/hb-ot-shape-complex-arabic.cc

diff --git a/src/hb-ot-shape-complex-arabic.cc b/src/hb-ot-shape-complex-arabic.cc
index 880a6b9..746f2d7 100644
--- a/src/hb-ot-shape-complex-arabic.cc
+++ b/src/hb-ot-shape-complex-arabic.cc
@@ -25,6 +25,7 @@
  */
 
 #include "hb-ot-shape-complex-private.hh"
+#include "hb-ot-shape-private.hh"
 
 
 
@@ -248,7 +249,7 @@
 
   for (unsigned int i = 0; i < count; i++)
   {
-    unsigned int this_type = get_joining_type (buffer->info[i].codepoint, (hb_unicode_general_category_t) buffer->info[i].general_category());
+    unsigned int this_type = get_joining_type (buffer->info[i].codepoint, _hb_glyph_info_get_general_category (&buffer->info[i]));
 
     if (unlikely (this_type == JOINING_TYPE_T)) {
       buffer->info[i].arabic_shaping_action() = NONE;